Reference signal distribution in multi-module systems

ABSTRACT

Methods and systems for reducing and eliminating the effects of reference signal leakage in multi-module radio-frequency systems, particularly for radio-frequency integrated-circuit devices. Isolation is provided by frequency shifting, by binary phase-shift keying of reference signals to furnish different reference signals with unique signatures to enable separation and discrimination, and by backwards sending of reference signals from receiver to transmitter to avoid contamination of receiver signals by leakage of transmitter reference signals. In addition to improving calibration, methods of the present invention also provide noise reduction in certain multiple-module radio frequency systems.

This application is a Divisional Application of U.S. patent applicationSer. No. 15/473,884, filed Mar. 30, 2017, titled ‘Reference signaldistribution in multi-module systems’, which is a continuation of PCTInternational Application No. PCT/II2015/050973, International FilingDate Sep. 24, 2015, claiming priority of U.S. Provisional PatentApplication No. 62/057,286, filed Sep. 30, 2014.

FIELD

The present invention is directed to multi-module radio-frequencycalibration, in particular to the calibration of radio frequencyintegrated circuits (RFIC).

BACKGROUND

Multi-module systems typically require sharing of frequency and phasereference signals for real-time calibration. In such systems, it isdesirable to measure transmission characteristics betweenarbitrarily-selected ports of the modules. For example, in aphased-array radar system it is necessary to know the relative phasecharacteristics at the respective antennas in order to be able to directa phased beam in a particular direction. In another example, multipleinput/multiple output (MIMO) radar systems require referencing receivedsignals to one another.

Under ideal conditions, measurement of reference signals is generallystraightforward. When transmission losses are high, however, signalleakage among module ports interferes with reference measurement. Forexample, when making multi-port measurements with a vector networkanalyzer (VNA) there is typically some signal leakage between the VNA'sports, which limits the dynamic range of the measurements. This problemis particularly pronounced in the case of a single RFIC, where theisolation is limited because of the small inter-port distances and theinherently-restricted isolation of the RFIC, the package, and theprinted circuit board (PCB). Here, the likely limit for isolation is onthe order of 50 dB, achieved between the most distantly-separated RFICports.

A known improvement to the above-described isolation problem is to use aseparate shielded RFIC for each port. In this way, the signaltransmitted to the device (or medium) under test (hereinafter denoted as“DUT”) has a significantly better isolation, and only the signal passingthrough the DUT reaches the other RFIC. Unfortunately, however, thisintroduces the problem of providing a phase reference to the mated RFIC.RFICs may have distinct synthesizers, so the phase of a signal from oneRFIC downconverted within another RFIC cannot be directly measured—onlycomparative measurements can be made. This requires that a sample of thereference signal be provided to the receiving RFIC. The straightforwardapproach for providing the reference is to bring a sample of thetransmitted signal to the receiving RFIC via a receiving port, and thenmeasure the phase difference between the signal from the DUT and thereference signal from the transmitting RFIC. However, bringing a signalat the test frequency can contaminate the signal from the DUT, becausethe receiving RFIC has limited isolation. The problem could be lessenedby weakening the reference signal, but doing so also reduces measurementaccuracy because of the degraded signal-to-noise ratio of the reference.

Under the conditions and restrictions described above, it would bedesirable to have methods for reducing or eliminating signal leakage;reducing or eliminating the affects of signal leakage on measurements;and making accurate measurements in spite of signal leakage. These goalsare met by embodiments of the present invention.

SUMMARY

Various embodiments of the present invention provide efficient andordered distribution of reference signals in RF systems having multiplereceivers and transmitters. These embodiments provide reference sharingamong the different ports of the modules, in configurations including,but not limited to: a star coupler featuring all-to-all referencecoupling; and neighboring module-to-module reference sharing.

In addition, certain embodiments of the present invention provideisolation for reference signals that are being shared among modules, byfurnishing each reference signal with a unique signature, allowingindividual reference signals to be identified and separated as necessarythroughout the system. According to various embodiments of theinvention, signatures can be applied via frequency-shifting or binaryphase-shift encoding.

Therefore, according to an embodiment of the present invention there isprovided a radio-frequency transmitter-receiver system including: (a) atransmitter for transmitting a transmitted signal at a transmissionfrequency; (b) a receiver for receiving the transmitted signal as areceived transmitted signal, wherein: (c) the receiver includes a localoscillator having a local oscillator signal at a local oscillatorfrequency, for downconverting the received transmitted signal from thetransmission frequency to an intermediate frequency as a receiverintermediate frequency signal; (d) a transmitter downconverterassociated with the transmitter, for downconverting the transmittedsignal from the transmission frequency to the intermediate frequency asa transmitter intermediate frequency signal; (e) a reference signal pathfrom the local oscillator to the transmitter downconverter, forconveying the local oscillator signal from the local oscillator to thetransmitter downconverter; and (f) a phase comparator, for measuring aphase difference between the receiver intermediate frequency signal andthe transmitter intermediate frequency signal.

In addition, according to another embodiment of the present invention,there is also provided a method for calibrating a radio-frequencytransmitter-receiver system having a transmitter with a transmittedsignal, a receiver with a local oscillator signal and a receiverintermediate frequency signal, and a transmitter downconverter, themethod including: (a) downconverting the transmitted signal via thedownconverter to a transmitter intermediate frequency signal accordingto the local oscillator signal; (b) measuring a phase difference betweenthe receiver intermediate frequency signal and the transmitterintermediate frequency signal; and (c) calibrating thetransmitter-receiver system according to the phase difference.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter disclosed may best be understood by reference to thefollowing detailed description when read with the accompanying drawingsin which:

FIG. 1 is a top-level block diagram of a radio-frequency integratedcircuit (RFIC).

FIG. 2 is a block diagram of an exemplary transmit/receive module.

FIG. 3 is a block diagram of a system of two RFICs.

FIG. 4A is a block diagram of a multi-module RFIC system havingbidirectional ports used for calibration.

FIG. 4B is a block diagram of a multi-module RFIC system havingdedicated ports used for calibration.

FIG. 5A is a block diagram showing a reference signal sent from atransmitting module to a receiving module.

FIG. 5B is a block diagram showing a reference signal sent from areceiving module to a transmitting module.

FIG. 6A is a schematic diagram of a simplified all-to-all symmetric starcoupler.

FIG. 6B is a block diagram of a symmetric all-to-all reference signaldistributor.

FIG. 6C is a block diagram of an exemplary 8-port all-to-allButler/Hadamard coupler.

For simplicity and clarity of illustration,elements shown in the figuresare not necessarily drawn to scale, and the dimensions of some elementsmay he exaggerated relative to other elements. In addition, referencelabels may be repeated among the figures to indicate corresponding oranalogous elements.

DETAILED DESCRIPTION

Certain embodiments of the present invention provide a reference signalthat is distinct from the transmitted signal, so that leakage of thereference signal into the signal from the DUT does not cause ameasurement error. In some of these embodiments, the reference signal ismathematically orthogonal to the transmitted signal. According to arelated embodiment, orthogonality is attained by frequency offsetting;according to another related embodiment, orthogonality is attained bybinary phase shift keying (BSPK), either fast or slow.

In one embodiment, the transmitting RFIC conveys the reference to otherRFICs using BPSK (a non-limiting example of which uses 1 MHzmodulation). The signal to the DUT is sent as a continuous wave (CW). Asa result, the reference signal contains no spectral component at 0 Hz(DC). The receiving RFICs receive both the CW signal from the DUT (onone of the ports) and the BPSK-modulated signal (on another port). Thereference signal is BPSK-demodulated, downconverted and integrated insoftware so as to obtain the reference phasor. Because the reference andthe DUT signals are orthogonal, there is no mutual contamination. TheBPSK modulation can be implemented through BPSK toggling at the TRmodule sending the reference (though it can introduce noise)—modulationthrough arbitrary waveform generation (AWG) is not an option since RFIC₁is dedicated to generating the transmitted CW signal arid RFIC₂ isdedicated to generating the CW receive signal. In an alternativeembodiment, BPSK modulation is performed on the local oscillator (LO)signal in the transmitting module receiving the reference.

In another embodiment of the present invention, the transmitting chipconveys the reference to the other chips using BPSK modulation on asnapshot-by-snapshot basis. One snapshot is taken with the referencesent at regular polarity and the other at opposite polarity. The twosnapshots are summed for the regular signal and subtracted for thereference signal. The snapshot can be halved in time to maintain sameresolution bandwidth (RBW). In a related embodiment, BPSK modulation isimplemented through software polarity toggling at the transmittingmodule (software-based toggling avoids injecting noise).

In a further embodiment, the transmitting RFIC sends to the receivingRFIC the receive local oscillator (RX_LO) as a reference rather than thetransmitted signal. The receiving RFIC is configured to a RX_LO′frequency which is offset from both the transmitted signal and the RX_LOfrequency. In a non-limiting example, the transmitting frequency is10.01.0 GHz, RX_LO is 10.008 GHz and RX_LO′ is 10.007 GHz. Then thetransmitting RFIC will receive the transmission at 2 MHz, while thereceiving RFIC will receive the transmission at 3 MHz and RX_LO at 1 MHzby digitally downconverting the 3 MHz with the received 1 MHzdownconverted RX_LO signal. In practice, this is done by multiplying thedownconverted 3 MHz, samples with conjugate of downconverted 1 MHzsamples. Because there is no signal conveyed to the receiving RFIC atthe transmitting frequency, the measurement of the signal from the DUTis not contaminated by leakage.

In the above example, if RX_LO′ is higher in frequency than RX_LO (e.g.LO_RX is 10.007 GHz and RX_LO′ is 10.008 GHz) the reference is convertedto a “negative frequency” (“−1 MHz”), and during the reconstruction noconjugation is needed.

In respective related embodiments, the above cases extend to arbitrarynumbers of multiple receive RFICs. Since only one RFIC transmits at anygiven time, the processed reference signals are distributed to the otherRFICs.

In further embodiments of the present invention, simultaneoustransmission is done from multiple RFICs. In a related embodiment,staggered frequencies (by an order of RBW) are used, where the RBWfrequency offset does not unsatisfactorily degrade the measurement.

Other embodiments provide BPSK manipulation of the transmitted signalsin cases where multiple reference signals need to be distributed. In arelated embodiment, BPSK encoding (such as by Hadamard matrix rows) oftransmission signals are used to distinguish between the multiplereference signals. In this embodiment, the encodings of the referencessignals of the RFICs are mutually orthogonal and thus distinguishable.The BPSK code [1 1 . . . 1] is not used, to avoid contaminating thetransmitted signal.

A further related embodiment provides multiple RX_LO frequencies, sothat the mixed frequency differences are distinct. In a non-limitingexample, 4 RFICs with RX_LO frequencies of F₀, F₀+df, F₀3 df and +4 df;respectively, can be used. The df coefficients 0, 1, 3, and 4 are chosento avoid overlaps caused by the oscillator×mixing. That is, RFIC, willreceive at frequencies +df, +3 df and +4 df; RFIC₂ will receive thereferences at −df +2 df and +3 df, etc., such that all absolute valuesare distinct. The scheme can be further extended—for example for 8 RFICsthe frequency offsets could be [0,1,3,4,9,10,12,13]df. This techniquecan be used in conjunction with using the same frequency fortransmission and as a local oscillator in each module (such as in FMCW,CW or stepped CW radar), so that each module can receive all the rest ofthe modules and distinguish between their signals, both in the referencepath and in the over-the-air path.

Another embodiment provides orthogonal multiplexing for multi-moduleoperation when several modules are transmitting. This embodimentachieves not only the benefits of reference signal isolation, but also atime- and memory-efficient multiple operation per sweep, such as for amultistatic radar application.

An additional embodiment of the present invention further provides asolution to a problem which arises when demodulating a received signalwith a signal derived from the same LO as the transmitted signal. Anyspur or artifact situated about the LO contributes to an effective noisefloor. Examples of such artifacts include: the image componentsassociated with quadrature modulation imbalance and reference spurssituated about the LO. According to this embodiment of the invention,the problem may be avoided in the multi-module case by shifting the LOof each module relative to all others, thus also shifting the associatedartifacts.

A further embodiment of the present invention circumvents the need tosend a replica of the transmitted signal to the receiving module.Conveying a replica of the transmitted signal to the receiving moduleallows characterizing the relative phase between the reference signalpath and the signal path through the device or medium under test bymeasuring the relative phase of intermediate frequency signals resultingfrom the mixing of the received signal with a local oscillator. However,any leakage of the replica of the transmitted signal in the receiver cancontaminate the received signal. A related embodiment of the presentinvention avoids this problem by avoiding sending a replica of thetransmitted signal to the receiving module. Instead, this embodimentprovides a replica of the receiving module's local oscillator (LO) backto the transmitting module. The transmitting module then locally mixes(via a dedicated downconverter) the receiving module's LO with thetransmitted signal, thereby generating an intermediate frequency (IF) atthe transmitter. This transmitter intermediate frequency signal isindicative of the relative phase between the transmitted signal and thereceiver's intermediate frequency signal. By performing this operationin the transmitting module rather than in the receiving module, thisembodiment guarantees that only the transmitted signal passing throughthe DUT reaches the receiving module, and that no other signal at samefrequency is present there.

What is claimed is:
 1. A method for calibrating a radio-frequencytransmitter-receiver system having a transmitter with a transmittedsignal, a receiver with a receiver intermediate frequency signal and alocal oscillator signal as a reference signal, and a transmitterdownconverter, the method comprising: downconverting the transmittedsignal via the downconverter to a transmitter intermediate frequencysignal according to the local oscillator signal; furnishing thereference signal with a unique signature, allowing the reference signalto be identified; measuring a phase difference between the receiverintermediate frequency signal and the transmitter intermediate frequencysignal; and calibrating the transmitter-receiver system according to thephase difference.
 2. The method of claim 1, wherein the unique signatureis applied via frequency-shifting.
 3. The method of claim 1, wherein theunique signature is applied via binary phase-shift encoding.
 4. Aradio-frequency transmitter-receiver system comprising: at least onetransmitting module for transmitting a transmitted signal at a sharedtransmission frequency, and including a transmitter downconverter fordownconverting the transmitted signal to a transmitter intermediatefrequency signal; at least one receiving module for receiving thetransmitted signal as a received transmitted signal, each at least onereceiving module including a local oscillator having a local oscillatorsignal at a shared local oscillator frequency, for downconverting thereceived transmitted signal from the shared transmission frequency to anintermediate frequency as a receiver intermediate frequency signal; anda radio-frequency integrated circuit; wherein: each local oscillatorsignal of the at least one receiving module has a unique signature; theradio-frequency integrated circuit is operative to: identify a localoscillator signal according to the unique signature thereof; measure aphase difference between a receiver intermediate frequency signal and atransmitter intermediate frequency signal according to the uniquesignature thereof; and calibrate the radio-frequencytransmitter-receiver system according to the phase difference.
 5. Theradio-frequency transmitter-receiver system of claim 4, comprising aplurality of transmitting modules and a plurality of receiving modules,wherein the radio-frequency integrated circuit is further operative toseparating local oscillator signals according to the respective uniquesignatures thereof.
 6. The radio-frequency transmitter-receiver systemof claim 4, wherein a unique signature is applied viafrequency-shifting.
 7. The radio-frequency transmitter-receiver systemof claim 4, wherein a unique signature is applied via binary phase-shiftencoding.